This invention relates to a controllable rectifier circuit and, more particularly, to such a controllable rectifier circuit which can be used in a power supply and which is capable of producing either a half-wave or a full-wave rectified signal as a function of a control signal supplied to the circuit.
In a typical power supply, for example, a supply for producing DC power from AC power, a rectifier is used to convert an input AC signal to a DC signal. Generally, such a rectifier is a full wave rectifier formed of, for example, a bridge rectifier circuit. As is known, the bridge rectifier rectifies both the positive and negative half cycles of the AC signal so as to produce successive, rectified half cycles. Of course, the DC power which is derived from the full wave rectifier is greater than that which would be derived if the rectifier was a half wave rectifier whereby only alternate half cycles of the AC signal would be rectified and supplied to a load. There are many applications wherein a particular rectifier, such as a bridge rectifier, should be controlled to vary the amount of power which is supplied thereby. Thus, if it is desired to utilize the rectifier to produce a lower power level, it is convenient to operate that rectifier as a half wave rectifier. When that same rectifier is to supply a higher power level, then it should be operated as a full wave rectifier.
One application of such a controllable rectifier circuit that is convertible from half wave rectification to full wave rectification is in supplying power to an induction heating device. In such a device, an induction element, typically a coil, is supplied with relatively high frequencies to generate an alternating flux. If a conducting material is placed in this flux, eddy currents are induced therein, and these induced eddy currents produce heat. The amount of heat is a function of the frequency of the flux as well as the magnitude thereof. The magnitude of the flux is a function of the power supplied to the induction element, and the frequency at which the flux changes is determined by opening and closing a switching device which is connected in series with the element. Thus, the amount of heat which is produced by the induction heating device easily can be controlled by controlling the frequency at which the switching device operates and by controlling the amount of power which is supplied to the induction heating device.
In one type of control circuit for an induction heating device, a controllable oscillator is provided to drive the switching device, the frequency of this oscillator being controlled continuously by an adjustable element, such as a potentiometer. Thus, when the amount of heat is to be increased, the setting of the potentiometer is changed so as to provide a continuous, gradual change in the oscillator frequency, thus changing the switching frequency of the switching device and the rate of change of the flux which is generated by the induction element. For lower heating levels, the current which is supplied to the induction heating element is a half wave rectified AC current. Thus, the magnitude of the flux which is generated by the induction element is relatively low. When the heating level is to be increased to a higher range, the power supply is operated as a full wave rectifier to supply full wave rectified current to the induction element. This, of course, increases the magnitude of the flux generated thereby. In order to provide a continuous increase in the heating levels over the lower and higher ranges, it is usual to return the frequency of the controllable oscillator to its initial frequency at the time that the conversion is made from half wave rectification to full wave rectification, and then to gradually change the frequency of the oscillator once again.
One advantage in changing both the frequency and magnitude of the generated flux rather than just the frequency thereof is that a relatively large heat controlling range can be obtained while using only a relatively narrow frequency range. Since a smaller frequency range is needed, the noise frequencies which are radiated from the induction heating device also are limited to a smaller range. Furthermore, since the controllable oscillator need be varied over only a relatively narrow frequency range, it can be of simpler construction and, therefore, of lower cost. Also, the switching device which is used to interrupt the rectified current flowing the induction element is not very expensive.
One technique in operating a bridge rectifier either as a half wave rectifier or as a full wave rectifier is to provide two of the arms of the rectifier, and thus both of the current paths therein, with switchable rectifiers, such as thyristors. If only one of the thyristors is rendered conductive, then only one of the half cycles of the input AC signal will be rectified, thus resulting in half wave rectification thereof. On the other hand, if both thyristors are rendered conductive, then first one current path will conduct one half cycle and then the other current path will conduct the other half cycle of the AC signal, thus resulting in full wave rectification. It is important to trigger the thyristors into conduction at the beginning of the respective half cycle which is conducted thereby. If trigger pulses are generated at the beginning of each half cycle, such trigger pulses may overlap the ending portion of the previous half cycle and the beginning portion of the next half cycle. For full wave rectification, this presents no problem because both half cycles are conducted, in sequence, by the respective thyristors. However, this overlap in the trigger pulses is not desired when half wave rectification is obtained. For example, let it be assumed that only the positive half cycles are to be conducted. This means that the thyristor which is included in the positive current path will be triggered into conduction only at the beginning of the positive half cycle, even if the trigger pulse applied thereto overlaps with the ending portion of the negative half cycle. However, if this same trigger pulse is supplied to the thyristor which is included in the negative current path, then this thyristor will be conductive during the ending portion of the negative half cycle because of such a trigger pulse. Consequently, the half wave rectified current which is produced appears as alternate positive half cycles with a noise pulse superimposed onto the beginning of each positive half cycle. Such noise pulses may result in the generation of undesired, radiated noise and, moreover, may damage the elements which constitute the induction heating device.